1. Field of the Invention PA0 2. Description of the Prior Art
The present invention relates to a method for injecting a material, such as plasticized synthetic resin etc., to be molded into an injection mold having its interior held under a high vacuum and molding the material, and to an apparatus therefor.
Matters of primary concern in the injection molding art to date are how the reproducibility in articles molded by an injection mold can be enhanced to the full extent and how the molding cycle can be shortened. Conventional injection molding methods have disadvantages with respect to the reproducibility that it is impossible to obtain accuracy in dimensions on the order of microns, that there still remains a problem of deformation and that a mold joint line is inevitably produced which detracts from the appearance.
With respect to the accuracy in dimensions, at first, the difference in dimension between an article to be obtained and an actually obtained article falls in the range of from 10 to 20 microns. This is caused by exhaust disposition of an injection mold.
There have heretofore been proposed injection molding methods and apparatuses utilizing an injection mold which has its interior vacuumized prior to an injection process. Japanese Patent Public Disclosure No. Sho 56(1981)-123841, for example, aims at mere degassing of the interior of an injection mold by provision of an exhaust and cannot bring the injection mold interior to a state of a high vacuum. In providing a vacuumizing process during the injection molding, the time for exhaust of gas is limited to a given length of time in the molding cycle. In Japanese Utility Model Public Disclosure Nos. Sho 55(1980)-151522 and Sho 56(1981)-19317, a suction port is provided such as in the parting surface of the injection mold. The suction port is formed in the shape of a critical slit (minute aperture) which is generally 10-20 microns in diameter. For this reason, the suction port would require much time to bring the interior of the injection mold to a highly vacuumized state and thus, during a given length of time in the molding cycle, serves merely as a vent for gases. The higher the degree of vacuum, the longer the mean free path of the gas molecules. Therefore, the critical slit if it is used for suction produces great resistance to gas flow and can therefore not sufficiently fulfill the function as a suction port. For example, the diameter of the suction port is required to be about 50 microns so as to cause the injection mold interior to quickly be brought to a pressure below 1 torr and, in order to cause the internal pressure to become 10.sup.-1 torr, the diameter is required to be about 0.5 mm. That is to say, the diameter of a suction port for obtaining a high vacuum must be large enough to correspond to a purpose to be attained, and a special construction is required to achieve the purpose. It is impossible, however, to form such a special construction on the parting surfaces of the mold members of an injection mold which have been closed together and, in order to form such a special construction on the surface of the injection mold in which a cavity is formed, a further complicated construction is required.
Secondly, some kind of deformation is inevitably given to a molded article because the pressure during injection is large and because the temperature of the injection mold is high. High-speed injection is required in order to shorten the molding cycle. However, since high-speed injection has not yet been accomplished, it is necessary to increase the pressure during injection. It is a common knowledge in the art that injection is effected under high pressure and that the pressure within the injection mold (hereinafter referred to simply as "internal pressure") is in the range of from about 500 kg/cm.sup.2 to about 1500 kg/cm.sup.2, although there has been made an attempt to carry out the molding under ultrahigh pressure of not less than 2500 kg/cm.sup.2 so as to make the shrinkage of a material to be molded as small as possible and enhance the accuracy of the dimensions. As a result of such injections, residual stress is exerted on the molded article, giving rise to deformation, such as internal distortion, warps, etc. In view of the fact that the higher the pressure, the larger the distortion of the injection mold, it is necessary to pay attention to structure and strength of the injection mold. This is particularly true in molding which requires accuracy of the article to be obtained, since the injection mold has to be regarded as a precise pressure container taking into consideration the pressure during injection, and various restrictions are imposed on the structure of the injection mold, making it difficult to design a desired injection mold. Further, to allow the injection mold to endure high pressure, the mechanism for clamping the injection mold tends to become large-scale installation. Thus, the requirement that the pressure during injection be increased due to the impossibility of high-speed injection causes a vicious circle and, therefore, shortening of the molding cycle has its own limit.
Thirdly, since it is impossible to avoid change in viscosity of molten resin during the injection process according to the conventional injection techniques, it is impossible to prevent a mold line from being produced. The elevation in temperature of the injection mold to eliminate the production of a mold line is unfavorable because the time for cooling the injection mold is lengthened and because the molten resin becomes inferior.